Time-Of-Day Effects on Calf-Muscle Electromyography During Static Single-Leg Balance in Young Active Males

Introduction. Circadian rhythms influence neuromuscular performance, yet their effect on postural muscle activation during balance tasks remains unclear. This study aimed to determine time-of-day differences in calf-muscle EMG activity during static single-leg stance. Material and Methods. Ten healthy, physically active males (24.8 ± 0.8 years) performed three 15-s single-leg balance tests at 07:00-07:30 and 17:00-17:30 on the same day. EMG signals were recorded according to SENIAM standards, normalized to maximal voluntary isometric contractions (MVICs), and evaluated as a percentage of MVIC (%MVC). Paired t-tests and effect sizes (Cohen’s dz) were used to compare sessions. Results. EMG amplitudes were consistent in two time points. Mean differences between morning and evening were minimal (Soleus: –0.12 μV; Medial gastrocnemius: 0.13 μV; Lateral gastrocnemius: 0.83 μV), with 95% confidence intervals crossing zero and small effect sizes (dz < 0.30). Conclusions. The EMG activity of the calf muscles does not change with the time of day in young, physically active men. These findings support the physiological stability of tonic postural muscles under low-intensity conditions and suggest that balance training or rehabilitation tasks of this type can be performed flexibly throughout the day without affecting neuromuscular engagement.